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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="unordered.compliance"></a><a class="link" href="compliance.html" title="Standard Compliance">Standard Compliance</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="compliance.html#unordered.compliance.move">Move emulation</a></span></dt>
<dt><span class="section"><a href="compliance.html#unordered.compliance.allocator_compliance">Use of allocators</a></span></dt>
<dt><span class="section"><a href="compliance.html#unordered.compliance.construction">Construction/Destruction
      using allocators</a></span></dt>
<dt><span class="section"><a href="compliance.html#unordered.compliance.pointer_traits">Pointer Traits</a></span></dt>
<dt><span class="section"><a href="compliance.html#unordered.compliance.pairs0">Pairs</a></span></dt>
<dt><span class="section"><a href="compliance.html#unordered.compliance.misc">Miscellaneous</a></span></dt>
</dl></div>
<p>
      The intent of Boost.Unordered is to implement a close (but imperfect) implementation
      of the C++17 standard, that will work with C++98 upwards. The wide compatibility
      does mean some comprimises have to be made. With a compiler and library that
      fully support C++11, the differences should be minor.
    </p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="unordered.compliance.move"></a><a class="link" href="compliance.html#unordered.compliance.move" title="Move emulation">Move emulation</a>
</h3></div></div></div>
<p>
        Support for move semantics is implemented using Boost.Move. If rvalue references
        are available it will use them, but if not it uses a close, but imperfect
        emulation. On such compilers:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            Non-copyable objects can be stored in the containers. They can be constructed
            in place using <code class="computeroutput"><span class="identifier">emplace</span></code>,
            or if they support Boost.Move, moved into place.
          </li>
<li class="listitem">
            The containers themselves are not movable.
          </li>
<li class="listitem">
            Argument forwarding is not perfect.
          </li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="unordered.compliance.allocator_compliance"></a><a class="link" href="compliance.html#unordered.compliance.allocator_compliance" title="Use of allocators">Use of allocators</a>
</h3></div></div></div>
<p>
        C++11 introduced a new allocator system. It's backwards compatible due to
        the lax requirements for allocators in the old standard, but might need some
        changes for allocators which worked with the old versions of the unordered
        containers. It uses a traits class, <code class="computeroutput"><span class="identifier">allocator_traits</span></code>
        to handle the allocator adding extra functionality, and making some methods
        and types optional. During development a stable release of <code class="computeroutput"><span class="identifier">allocator_traits</span></code> wasn't available so an
        internal partial implementation is always used in this version. Hopefully
        a future version will use the standard implementation where available.
      </p>
<p>
        The member functions <code class="computeroutput"><span class="identifier">construct</span></code>,
        <code class="computeroutput"><span class="identifier">destroy</span></code> and <code class="computeroutput"><span class="identifier">max_size</span></code> are now optional, if they're not
        available a fallback is used. A full implementation of <code class="computeroutput"><span class="identifier">allocator_traits</span></code>
        requires sophisticated member function detection so that the fallback is
        used whenever the member function call is not well formed. This requires
        support for SFINAE expressions, which are available on GCC from version 4.4
        and Clang.
      </p>
<p>
        On other compilers, there's just a test to see if the allocator has a member,
        but no check that it can be called. So rather than using a fallback there
        will just be a compile error.
      </p>
<p>
        <code class="computeroutput"><span class="identifier">propagate_on_container_copy_assignment</span></code>,
        <code class="computeroutput"><span class="identifier">propagate_on_container_move_assignment</span></code>,
        <code class="computeroutput"><span class="identifier">propagate_on_container_swap</span></code>
        and <code class="computeroutput"><span class="identifier">select_on_container_copy_construction</span></code>
        are also supported. Due to imperfect move emulation, some assignments might
        check <code class="computeroutput"><span class="identifier">propagate_on_container_copy_assignment</span></code>
        on some compilers and <code class="computeroutput"><span class="identifier">propagate_on_container_move_assignment</span></code>
        on others.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="unordered.compliance.construction"></a><a class="link" href="compliance.html#unordered.compliance.construction" title="Construction/Destruction using allocators">Construction/Destruction
      using allocators</a>
</h3></div></div></div>
<p>
        The following support is required for full use of C++11 style construction/destruction:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            Variadic templates.
          </li>
<li class="listitem">
            Piecewise construction of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>.
          </li>
<li class="listitem">
            Either <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator_traits</span></code> or expression SFINAE.
          </li>
</ul></div>
<p>
        This is detected using Boost.Config. The macro <code class="computeroutput"><span class="identifier">BOOST_UNORDERED_CXX11_CONSTRUCTION</span></code>
        will be set to 1 if it is found, or 0 otherwise.
      </p>
<p>
        When this is the case <code class="computeroutput"><span class="identifier">allocator_traits</span><span class="special">::</span><span class="identifier">construct</span></code>
        and <code class="computeroutput"><span class="identifier">allocator_traits</span><span class="special">::</span><span class="identifier">destroy</span></code> will always be used, apart from
        when piecewise constructing a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>
        using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span></code> (see <a class="link" href="compliance.html#unordered.compliance.pairs">below</a>),
        but that should be easily avoided.
      </p>
<p>
        When support is not available <code class="computeroutput"><span class="identifier">allocator_traits</span><span class="special">::</span><span class="identifier">construct</span></code>
        and <code class="computeroutput"><span class="identifier">allocator_traits</span><span class="special">::</span><span class="identifier">destroy</span></code> are never called.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="unordered.compliance.pointer_traits"></a><a class="link" href="compliance.html#unordered.compliance.pointer_traits" title="Pointer Traits">Pointer Traits</a>
</h3></div></div></div>
<p>
        <code class="computeroutput"><span class="identifier">pointer_traits</span></code> aren't used.
        Instead, pointer types are obtained from rebound allocators, this can cause
        problems if the allocator can't be used with incomplete types. If <code class="computeroutput"><span class="identifier">const_pointer</span></code> is not defined in the allocator,
        <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">pointer_to_other</span><span class="special">&lt;</span><span class="identifier">pointer</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">value_type</span><span class="special">&gt;::</span><span class="identifier">type</span></code>
        is used to obtain a const pointer.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="unordered.compliance.pairs0"></a><a name="unordered.compliance.pairs"></a><a class="link" href="compliance.html#unordered.compliance.pairs0" title="Pairs">Pairs</a>
</h3></div></div></div>
<p>
        Since the containers use <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>
        they're limited to the version from the current standard library. But since
        C++11 <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>'s <code class="computeroutput"><span class="identifier">piecewise_construct</span></code>
        based constructor is very useful, <code class="computeroutput"><span class="identifier">emplace</span></code>
        emulates it with a <code class="computeroutput"><span class="identifier">piecewise_construct</span></code>
        in the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered</span></code> namespace. So for example, the
        following will work:
      </p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_multimap</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">;</span>

<span class="identifier">x</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered</span><span class="special">::</span><span class="identifier">piecewise_construct</span><span class="special">,</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_tuple</span><span class="special">(</span><span class="string">"key"</span><span class="special">),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_tuple</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">));</span>
</pre>
<p>
        Older drafts of the standard also supported variadic constructors for <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>,
        where the first argument would be used for the first part of the pair, and
        the remaining for the second part.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="unordered.compliance.misc"></a><a class="link" href="compliance.html#unordered.compliance.misc" title="Miscellaneous">Miscellaneous</a>
</h3></div></div></div>
<p>
        When swapping, <code class="computeroutput"><span class="identifier">Pred</span></code> and
        <code class="computeroutput"><span class="identifier">Hash</span></code> are not currently swapped
        by calling <code class="computeroutput"><span class="identifier">swap</span></code>, their copy
        constructors are used. As a consequence when swapping an exception may be
        thrown from their copy constructor.
      </p>
<p>
        Variadic constructor arguments for <code class="computeroutput"><span class="identifier">emplace</span></code>
        are only used when both rvalue references and variadic template parameters
        are available. Otherwise <code class="computeroutput"><span class="identifier">emplace</span></code>
        can only take up to 10 constructors arguments.
      </p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003, 2004 Jeremy B. Maitin-Shepard<br>Copyright © 2005-2008 Daniel
      James<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
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